Gas detecting systems have been available in the prior art in which a plurality of sensors are used, each sensor having a different response characteristic from the other. In U.S. Pat. No. 4,638,443, a gas detecting apparatus is disclosed that determines a ratio of conductivity of each sensor as it becomes exposed to the gas sample. Various mathematical formula are used, including logarithmic operations, to obtain a linear relationship of the conductivity ratio versus the concentration of the particular gas in question. This system then finds "detection patterns," and calculates a "degree of similarity." Once this information has been derived, the gas detecting apparatus can discern the presence of the particular target gas by use of the detection patterns and degree of similarity.
In U.S. Pat. No. 4,542,640 an array of gas sensors is used each having a different response to the selected gases. In this system, at least as many sensors are required as different gases to be detected. The various response patterns of the array of sensors are used to create simultaneous equations, which then can be solved to determine the presence of the selected target gases.
In U.S. Pat. No. 4,457,161, a gas detection system is disclosed in which a sensor array is used to detect concentrations of multiple types of gases. Each sensor has a different sensitivity to the particular target gases, and the sensor's voltage outputs can be quantified by use of simultaneous linear equations.
In U.S. Pat. No. 4,818,348 a system for identifying and quantifying simple and complex chemicals is disclosed. The materials to be analyzed are heated and vaporized. A sensor array is used in which each sensor has a response characteristic that is different from the other sensors. A response pattern is created and compared to a stored pattern to identify the chemical of interest, and this information is independent of concentration. The concentration can then be determined by solving simultaneous equations.
A major problem with conventional chemical analysis systems is that they really do not work well at all. In order to achieve workable simultaneous equations that can be analyzed to find either the presence of a target gas or the concentration of the target gas, the response of the individual sensors in the array of sensors must be very repeatable, and must not drift over time. This is difficult enough to achieve with one or two sensors, and is nearly impossible when using high numbers of sensors (e.g. five or more sensors). Conventional systems that can analyze the presence or concentration of gases in an open area typically use five or more different sensors, in order to have enough information be somewhat reliable in predicting a minimum concentration of important or dangerous gases. The sensors must be calibrated very frequently, or else the calculations used to make the determination of the presence of a particular gas will become quite useless.
It would be desirable to provide a gas detecting system that did not require recalibration at very short time intervals. In addition, it would be desirable for such a system to use relatively few sensors, regardless of whether or not such sensors exhibit severe drift problems (whether such sensors are repeatable or stable over long periods of time). It would be even more desirable to correct for drift of sensors that are not repeatable or stable over long periods of time.